1. Field of the Invention
The present invention relates to a semiconductor device and a mode of operation for the semiconductor device. In particular, such devices utilize optical properties of charged excitons, and are related to optical devices including an optical modulator, an optical detector and an optical memory displaying bistability.
2. Description of the Related Art
An optical modulator is an electro-optic device where the intensity of a transmitted optical beam is switched between lower and higher values by controlling applied voltage. An efficient optical modulator must have a large contrast between the transparent and opaque states, as well as having low loss in the transparent state.
U.S. Pat. No. U.S. Pat. No. 4,872,744 describes and claims optical modulator comprising a quantum well layer and means in the form of a field effect transistor for altering the density of free carriers within the well layer. The field effect transistor is in the general form of a high electron mobility transistor (HEMT) but configured to guide light through a region containing the well layer. The bias applied to the gate electrode makes the device relatively transparent or opaque to the light in transit.
The device according to the aforementioned US Patent, when operated as described, still absorbs some radiation in the transparent state, even though it absorbs more in the opaque state. This has a number of disadvantages. First, it is impairs the contrast between the opaque and transparent states. Second, it results in higher power being required for the optical input beam. The absorption of the transparent state also restricts the overall length of a waveguide that can be formed containing the device as an integral element, which limits its use in an integrated optical circuit.
The present invention also relates to optically activated semiconductor devices in which an incident optical beam (hereinafter called a "control beam") is either used to generate an electrical signal or else to control the intensity or switching of an optical beam transmitted through the device. A device of the former kind is, of course, an optical detector and a device of the latter kind is an optically activated optical modulator.
The principle of controlling or switching a transmitted beam, using a control beam is described, for example, on U.S. Pat. No. 4,872,744. The control beam is said to change the electron density in a quantum well layer so that at zero--or low carrier density, the device is relatively opaque to a transmitted beam and at higher carrier densities, it is relatively transparent. However, no specific means of putting this into effect appears to be described.
Another device in which a control beam having a photon energy within a quantum well absorption band creates excess carriers within the well and so alters the refractive index of a layer is described in U.S. Pat. No. 4,626,075. This changes the angle deflection of a transmitted beam having a photon energy lower than the bandgap of the quantum well. Therefore, this kind of device acts as a phase modulator.
Another form of optically activated semiconductor device which has a waveguide with a P-I-N structure is described in U.S. Pat. No. 4,716,499. With this device, a control beam is used to generate a photo-voltage which in turn is used to modulate a transmitted beam by changing the biasing condition of the waveguide.
Furthermore, the present invention relates to a bistable optical device which is suitable for uses such as an element in an optical memory.
A semiconductor device which exhibits bistability in its output is disclosed in U.S. Pat. No. 4,716,499. This device relies on generation of a photocurrent to alter the biasing of the device for its bistable behavior.